Idle connection state power consumption reduction in a wireless local area network using beacon delay advertisement

ABSTRACT

A novel and useful apparatus for and method of improving idle connection state power consumption in wireless local area network (WLAN) system. Beacon transmission delay information is determined by the access points and advertised to the stations via a Beacon Transmission Delay Information Element. In response, the stations adjust their Wake For Beacon Reception time accordingly to wake up at a time much closer to the actual receipt of the Beacon, thereby reducing power consumption due to the reduced time the receive circuits need to be powered on.

REFERENCE TO PRIORITY APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 60/885,138, filed Jan. 16, 2007, entitled “Power Consumption in IdleConnection State”, incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of data communications andmore particularly relates to an apparatus for and method of improvingidle connection state power consumption in wireless local area network(WLAN) systems using beacon delay advertisements.

BACKGROUND OF THE INVENTION

Wireless Local Area Networks (WLANs) are well known in the art. Over thepast few years, wireless networking has exploded with numerous productscommercially available from a myriad of manufacturers. The standardsgoverning WLAN networking products are defined by a suite ofspecifications issued by the IEEE and known as the IEEE 802.11 standard,incorporated herein by reference in their entirety. The standards definethe operation of both the radio PHY layer and the MAC layer.

A wireless local area network (WLAN) links two or more computerstogether without using wires. WLAN networks utilize spread-spectrumtechnology based on radio waves to enable communication between devicesin a limited area, also known as the basic service set. This gives usersthe mobility to move around within a broad coverage area and still beconnected to the network.

For the home user, wireless networking has become popular due to theease of installation and location freedom with the large gain inpopularity of laptops. For the business user, public businesses such ascoffee shops or malls have begun to offer wireless access to theircustomers, whereas some are even provided as a free service. Inaddition, relatively large wireless network projects are beingconstructed in many major cities.

There are currently there exist several standards for WLANs: 802.11,802.11a, 802.11b, 802.11g and 802.11n. The 802.11b has a rate of 11 Mbpsin the 2.4 GHz band and implements direct sequence spread spectrum(DSSS) modulation. The 802.11a is capable of reaching 54 Mbps in the 5GHz band. The 802.11g standard also has a rate of 54 Mbps but iscompatible with 802.11b. The 802.11a/g implements orthogonal frequencydivision multiplexing (OFDM) modulation.

A WLAN state is any component that can connect into a wireless medium ina network. All stations are equipped with wireless network interfacecards (NICs) and are either access points or clients. Access points(APs) are base stations for the wireless network. They transmit andreceive radio frequencies for wireless enabled devices to communicatewith. Wireless clients can be mobile devices such as laptops, personaldigital assistants, IP phones or fixed devices such as desktops andworkstations that are equipped with a wireless network interface card.

The basic service set (BSS) is defined as the set of all stations thatcan communicate with each other. There are two types of BSS: (1)independent BSS and (2) infrastructure BSS. Every BSS has anidentification (ID) called the BSSID, which is the MAC address of theaccess point servicing the BSS. An independent basic service set (BSS)is an ad-hoc network that contains no access points, which means thestations within the ad-hoc network cannot connect to any other basicservice set.

A network diagram illustrating an example prior art WLAN network isshown in FIG. 1. The example network, generally referenced 10, comprisesWLAN access points 26, 32 (AP) coupled to a wired LAN 22 such as anEthernet network. The WLAN AP 26 in combination with laptops 28 formbasic service group (BSS) #1 24. Similarly, WLAN AP 32 in combinationwith laptop 34, personal digital assistant (PDA) 36 and cellphone 38,form basic service group #2 30. A server 12, desktop computers 14, 16,router 20 and Internet 18 are also connected to the wired LAN 22.

A block diagram illustrating an example prior art WLAN transceiver inmore detail is shown in FIG. 2. The WLAN transceiver, generallyreferenced 40, comprises antennas 42, 44, RF switch 46, I and Q signalanalog to digital converters (ADCs) 58, 60, respectively, I and Q signaldigital to analog converters (DACs) 61, 62, respectively, basebandprocessor PHY/MAC 69, EEPROM 63, static RAM 64, FLASH memory 65, hostinterface (I/F) 66 coupled to host 67 and power management circuit 68.Radio circuit 48 comprises bandpass filter 50, RF front end circuitry52, bandpass filter 54 and I/Q transceiver 56 that performs I and Qmodulation and demodulation.

A timing synchronization function (TSF) is operative to keep the timersof all the stations (STAs) in the same basic service set (BSS)synchronized. Each station maintains its own local TSF timer. In aconventional WLAN infrastructure network, the access point (AP) is thetiming master and is operative to implement the timing synchronizationfunction (TSF). The AP periodically transmits special frames calledbeacons that contain a copy of its TSF timer. The beacons are used bythe other STAs in the BSS to synchronize to the AP. A STA always acceptsthe timing information received in a beacon from the AP servicing itsBSS. If the TSF timer of a STA is different from the timestamp in thereceived beacon, the receiving STA sets its local timer to the receivedtimestamp value.

For ad hoc networks, the TSF in an Independent BSS (IBSS) is implementedusing a distributed algorithm that is performed by the members of theBSS. Each STA in the BSS transmits beacons in accordance with analgorithm defined in the 802.11 standard. Each STA in the IBSS adoptsthe timing received from any beacon or probe response that has a TSFvalue later than its own TSF timer. STAs expect to receive beacons at anominal rate. The interval between beacon transmissions is defined bythe aBeaconPeriod parameter of the STA. A STA sending a beacon sets thevalue of the timestamp to be equal to the value of the TSF timer of theSTA at the time that the first bit of the timestamp is transmitted tothe PHY plus the transmitting delays of the STA through its local PHYfrom the MAC-PHY interface to its interface with the wireless medium(i.e. antenna, etc.).

An infrastructure basic service set (BSS) can communicate with otherstations that are not in the same basic service set by communicatingthrough access points. An extended service set (ESS) is a set ofconnected BSSs. Access points in an ESS are connected by a distributionsystem. Each ESS has an ID called the SSID which is a 32-byte (maximum)character string. A distribution system connects access points in anextended service set. A distribution system is usually a wired LAN butcan also be a wireless LAN.

In infrastructure networks, the AP defines the timing for the entire BSSby transmitting beacons in accordance with the aBeaconPeriod attributewithin the AP. This define a series of target beacon transmission times(TBTTs) exactly aBeaconPeriod time units apart. Time zero is defined tobe a TBTT with the beacon being a delivery traffic indication message(DTIM) and transmitted at the beginning of a contention fee period(CFP). At each TBTT, the AP schedules a beacon as the next frame fortransmission. If the carrier sense mechanism determines that the mediumis busy, the AP delays the actual transmission of the beacon inaccordance with the basic medium access defined in the standard. Thebeacon period is adopted by all STAs when joining the BSS. A blockdiagram illustrating an example beacon transmission in a busy network isshown in FIG. 3.

Beacon generation in an IBSS ad hoc network is a distributed process.The beacon period is included in Beacon and Probe Response frames andSTAs adopt that beacon period when joining the IBSS. All members of theIBSS participate in beacon generation. Each STA maintains its own TSFtimer that is used for aBeaconPeriod timing. The beacon interval withinan IBSS is established b the STA that instantiates the IBSS. Thisdefines a series of TBTTs exactly aBeaconPeriod time units apart. Timezero is defined to be a TBTT. At each TBTT the STA (1) suspends thedecrementing of the backoff timer for any pending non-beacon or non-adhoc traffic indication (ATIM) transmission; (2) calculates a randomdelay uniformly distributed in the range between zero and twiceaCWmin×aSlotTime; (3) waits for the period of the random delay,decrementing the random delay timer using the same algorithm as forbackoff; (4) cancels the remaining random delay and the pending beacontransmission, if a beacon arrives before the random delay timer expires,and the ATM backoff timer resumes decrementing; and (5) sends a beaconif the random delay timer expires and no beacon has arrived during thedelay period.

Note that in an infrastructure network, the STAs always adopt the timerin a beacon or probe response from the AP in their BSS. In an IBSS, aSTA always adopts the information in the contents of the beacon or proberesponse when it contains a matching service set identifier (SSID) andthe value of the timestamp is later than the TSF timer of the STA (i.e.it adopts the timing of the fastest clock in the network).

The types of wireless LANs include peer to peer or ad-hoc wireless LANs.A peer-to-peer (P2P) WLAN enables wireless devices to communicatedirectly with each other. Wireless devices within range of each othercan discover and communicate directly without involving central accesspoints. This method is typically used by two computers so that they canconnect to each other to form a network. If a signal strength meter isused in this situation, it may not read the strength accurately and canbe misleading, because it registers the strength of the strongestsignal, which may be the closest computer.

The RF front end circuit 20 functions to filter and amplify RF signalsand perform RF to IF conversion to generate I and Q data signals for theADCs 26, 28 and DACs 30, 32. The baseband processor 34 is a part of thePHY that functions to modulate and demodulate I and Q data and carriersensing, transmission and receiving of frames. The medium accesscontroller (MAC) functions to control the communications (i.e. access)between the host device and applications. The power management circuit44 is adapted to receive power via a wall adapter, battery and/or powervia the host interface 42. The host interface may comprise PCI, CardBusor USB interfaces.

A problem associated with WLAN transceivers, however, is that theirpower consumption is a limiting factor in their deployment in mobilenetworks. WLAN transceivers consume relatively large amounts of powerfor the following reason. Wireless LAN transceivers are designed toserve computers throughout a structure with uninterrupted service usingradio frequencies. Due to the wide bandwidth used, the relatively highSNR required to demodulate the higher order WLAN constellations (64 QAM)and the possibility for strong adjacent channel signals, the transceiverhas to sample incoming signals at very high frequency (e.g., 4× orhigher then actual bandwidth) using high accuracy ADCs and highly linearreceiver chains, all of which consume high power.

In the majority of mobile use cases, a large percent of the time, themobile WLAN device is operating in the ‘idle’ receive mode. In thismode, the WLAN device is searching for and waiting to receive validpackets either from an access point (AP) or other stations (i.e. ad-hocnetwork). For active voice connections, the WLAN device is in the idlemode approximately 20-90% of the time, approximately 20-50% for standbyoperation and approximately 90% for scan operations.

While in the idle connection state, the STA is connected to the AP butvery little traffic flows, e.g., a packet is sent every few seconds. Inthis case, the STA is still required to wake up on DTIM/Listen intervalsto perform the following various activities: (1) receive broadcasttraffic, including NetBIOS name requests, ARP requests, UPnPadvertisements, etc.; (2) checking for unicast traffic destined for theSTA, including incoming call, application protocol messages, keyupdates, etc.; (3) perform management actions, including updating timingsynchronization function (TSF) values, tracking dynamic frequencyselection (DFS) (channel switch announcements); and (4) performing RXpath calibrations.

Most of the above actions require the STA to wake up and receive beaconmessages and process information received in the beacon message. A blockdiagram illustrating the multi-phase process of Beacon reception in aSTA is shown in FIG. 4. Beacon reception and related processing areperformed in three phases. In Phase 1 (block 162), the STA wakes upbefore the target beacon transmit time (TBTT) event to prepare forreception of the beacon. This phase comprises executing the wake-upsequence (block 164); switching on the RX chain (block 166); and waitingfor the arrival of the beacon message (block 168).

In Phase 2 (block 170), the STA receives the beacon message. The beaconis transmitted with some delay that is inherent to the design of the AP.The delay, however, is not known by the STA and may be hardware orsoftware based. Further, the beacon is transmitted at the lowest rate(e.g., 1 Mbps) to ensure reception with low SNR.

In Phase 3 (block 172), the receive beacon message is processed. Thisphase comprises switching off the RX chain (block 174); processing thecontents of the beacon message (block 176); and executing the dozecommand (block 178).

Standard WLAN implementations typically suffer from relatively high idlepower consumption (over 10% of the power consumed during activereception). This is because for idle mode operation they use thestandard radio receive circuit path which has relatively high powerconsumption associated with it. The majority of the power consumptionoccurs in the front end circuit, ADC circuits and the high speed digitalcorrelator logic circuits. Thus, considering the above described usagepatterns, idle power consumption constitutes the dominant part of thepower budget. In particular, maximal power consumption occurs while theRX chain is on.

It is thus desirable to have a mechanism that is capable of reducing orminimizing the power consumed while WLAN transceiver devices are in theidle connection state searching for WLAN beacon messages, signals, etc.In particular, optimization of the power consumption during the idleconnection state can significantly reduce the overall power consumptionof WLAN devices, improve standby and talk battery times and permit awider deployment in mobile devices.

SUMMARY OF THE INVENTION

The present invention is a novel and useful apparatus for and method ofimproving idle connection state power consumption in wireless local areanetwork (WLAN) systems. The present invention provides a mechanism forreducing the power consumption of stations (STAs) in a WLAN networkwhile in the Idle connection state.

Although the mechanism of the present invention can be used in numeroustypes of communication systems, to aid in illustrating the principles ofthe present invention, the description of the WLAN signal detectionmechanism is provided in the context of a WLAN radio co-located with aBluetooth radio that is part of a cellular phone.

In operation, mechanism determines the Beacon transmission delayinherent in its implementation. This delay is implementation specificand will vary from device to device and from manufacturer tomanufacturer. The delay can be determined at the time of manufacture bythe device vendor. The Beacon transmission delay information isadvertised by the access point to the stations via a special BeaconTransmission Delay Information Element. The stations, upon receipt,adjust their Wake For Beacon Reception time accordingly so that theywake up at a time much closer to the actual receipt of the Beacon, thusreducing power consumption due to the reduce time the receive circuitsneed to be on.

In addition, the mechanism takes advantage of the fact that not all thecontents of Beacon messages change from one Beacon to the next. Thus,stations only need receive the variable portion of the Beacon thatchanges. Access points determine the size of the variable portion oftheir Beacon messages and include this size information in a specialVariable Beacon Data Information Element in the Beacon message itself.The station reads the contents of this information element and uses thesize information to determine at what point it can halt the reception ofthe Beacon message and turn off it's receive radio thereby saving power.

Although the WLAN idle connection state power reduction mechanism of thepresent invention can be incorporated in numerous types of communicationdevices such a multimedia player, cellular phone, PDA, mobile device,etc., it is described in the context of a WLAN access point and station.It is appreciated, however, that the invention is not limited to theexample applications presented, whereas one skilled in the art can applythe principles of the invention to other communication systems as wellwithout departing from the scope of the invention.

The WLAN signal detection mechanism has several advantages including thefollowing: (1) use of the mechanism of the present invention providesfor a significant reduction in power consumption during the WLAN idlemode of operation which translates to over 300% of power savings forcommon usage scenarios of standby operation and 10% of power saving forvoice call in the enterprise environment and over 200% of power savingsfor common usage scenarios of standby operation and 4% of power savingfor voice call in the home environment; (2) depending on the particularimplementation, implementing the invention does not require additionalhardware; and (3) depending on the particular implementation, themechanism requires only software modifications to access points andstations.

Note that some aspects of the invention described herein may beconstructed as software objects that are executed in embedded devices asfirmware, software objects that are executed as part of a softwareapplication on either an embedded or non-embedded computer system suchas a digital signal processor (DSP), microcomputer, minicomputer,microprocessor, etc. running a real-time operating system such as WinCE,Symbian, OSE, Embedded LINUX, etc. or non-real time operating systemsuch as Windows, UNIX, LINUX, etc., or as soft core realized HDLcircuits embodied in an Application. Specific Integrated Circuit (ASIC)or Field Programmable Gate Array (FPGA), or as functionally equivalentdiscrete hardware components.

There is thus provided in accordance with the invention, a method foruse in an access point in a wireless local area network (WLAN) system,the method comprising the steps of measuring beacon transmission delayon the access point and advertising the measured beacon transmissiondelay in beacon messages thereby permitting recipient stations toschedule wake up times for beacon reception with significantly improvedaccuracy.

There is also provided in accordance with the invention, a method foruse in an access point in a wireless local area network (WLAN) system,the method comprising the steps of measuring beacon transmission delayon the access point from a target beacon transmission time (TBTT) eventto the time a beacon packet is forwarded to a channel access functionand advertising the measured beacon transmission delay in beaconmessages thereby permitting recipient stations to schedule wake up timesfor beacon reception with better accuracy.

There is further provided in accordance with the invention, a method foruse in an access point in a wireless local area network (WLAN) system,the method comprising the steps of determining a delay in transmissionof beacons beyond a time normally required for channel access andadvertising the measured beacon transmission delay in beacon messagesthereby permitting recipient stations to schedule wake up times forbeacon reception with better accuracy.

There is also provided in accordance with the invention, a method foruse in a station in a wireless local area network (WLAN) system, themethod comprising the steps of receiving a beacon message containingbeacon transmission delay information transmitted by an access point andupdating a wake for beacon reception time value in accordance with thebeacon transmission delay information.

There is further provided in accordance with the invention, a mobilecommunication device comprising a cellular radio, a WLAN radio, aprocessor coupled to the WLAN radio and the cellular radio, theprocessor operative to receive a beacon message containing beacontransmission delay information transmitted by an access point and updatea wake for beacon reception time value in accordance with the beacontransmission delay information.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a network diagram illustrating an example prior art WLANnetwork;

FIG. 2 is a block diagram illustrating an example prior art WLANtransceiver in more detail;

FIG. 3 is a block diagram illustrating beacon transmission in a busynetwork;

FIG. 4 is a block diagram illustrating the multi-phase process of Beaconreception in a STA;

FIG. 5 is an example communication device in more detail incorporatingthe WLAN idle connection state power reduction mechanism of the presentinvention;

FIG. 6 is a block diagram illustrating an example WLAN transceiverincorporating the idle connection state power reduction mechanism of thepresent invention;

FIG. 7 is a diagram illustrating the frame format for the beacontransmission delay information element of the present invention;

FIG. 8 is a flow diagram illustrating the method of advertising thebeacon transmission delay for use on an access point;

FIG. 9 is a flow diagram illustrating a first beacon transmission delaymeasurement method;

FIG. 10 is a flow diagram illustrating a second beacon transmissiondelay measurement method;

FIG. 11 is a flow diagram illustrating the method of advertising thebeacon transmission delay for use on a station;

FIG. 12 is a diagram illustrating the frame format for an example beaconhaving a variable beacon data information element of the presentinvention;

FIG. 13 is a flow diagram illustrating the method of beacon receptiontime reduction for use on an access point; and

FIG. 14 is a flow diagram illustrating the method of beacon receptiontime reduction for use on an access point.

DETAILED DESCRIPTION OF THE INVENTION Notation Used Throughout

The following notation is used throughout this document.

Term Definition AC Alternating Current ADC Analog to Digital ConverterAP Access Point ASIC Application Specific Integrated Circuit ATIM Ad HocTraffic Indication AVI Audio Video Interleave BMP Windows Bitmap BSSBasic Service Set BTD Beacon Transmission Delay CCK Complementary CodeKeying CFP Contention Fee Period CPU Central Processing Unit DAC Digitalto Analog Converter DC Direct Current DFS Dynamic Frequency SelectionDRAM Dynamic Random Access Memory DS Direct Sequence DSP Digital SignalProcessor DSSS Direct Sequence Spread Spectrum DTIM Delivery TrafficIndication Message EDCA Enhanced Distributed Channel Access EDR EnhancedData Rate EPROM Erasable Programmable Read Only Memory ERP Extended RatePhysical ESS Extended Service Set FCS Frame Check Sequence FM FrequencyModulation FPGA Field Programmable Gate Array GPS Ground PositioningSatellite HDL Hardware Description Language HT High Throughput IANAInternet Assigned Numbers Authority IBSS Independent Basic Service SetID Identification IE Information Element IEEE Institute of Electricaland Electronics Engineers IF Intermediate Frequency IP Internet ProtocolJPG Joint Photographic Experts Group LAN Local Area Network LBTD LastBeacon Transmission Delay MAC Media Access Control MBOA Multiband OFDMAlliance MP3 MPEG-1 Audio Layer 3 MPG Moving Picture Experts Group NICNetwork Interface Card OFDM Orthogonal Frequency Division MultiplexingOSI Open Systems Interconnection PC Personal Computer PCI PersonalComputer Interconnect PDA Portable Digital Assistant QAM QuadratureAmplitude Modulation RAM Random Access Memory RF Radio Frequency ROMRead Only Memory SDIO Secure Digital Input Output SIM SubscriberIdentity Module SNR Signal to Noise Ratio SRAM Static Random AccessMemory SSID Service Set Identifier STA Station TBTT Target BeaconTransmission Times TSF Timing Synchronization Function TV TelevisionUPnP Universal Plug and Play USB Universal Serial Bus UWB Ultra WidebandWiFi Wireless Fidelity WiMAX Worldwide Interoperability for MicrowaveAccess WiMedia Radio platform for UWB WLAN Wireless Local Area NetworkWMA Windows Media Audio WMV Windows Media Video WPAN Wireless PersonalArea Network

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a novel and useful apparatus for and method ofimproving idle connection state power consumption in wireless local areanetwork (WLAN) systems. The present invention provides a mechanism forreducing the power consumption of stations (STAs) in a WLAN networkwhile in the Idle connection state.

Although the mechanism of the present invention can be used in numeroustypes of communication systems, to aid in illustrating the principles ofthe present invention, the description of the WLAN signal detectionmechanism is provided in the context of a WLAN radio co-located with aBluetooth radio that is part of a cellular phone.

Although the WLAN idle connection state power reduction mechanism of thepresent invention can be incorporated in numerous types of communicationdevices such a multimedia player, cellular phone, PDA, mobile device,etc., it is described in the context of a WLAN access point and station.It is appreciated, however, that the invention is not limited to theexample applications presented, whereas one skilled in the art can applythe principles of the invention to other communication systems as wellwithout departing from the scope of the invention.

Note that throughout this document, the term communications device isdefined as any apparatus or mechanism adapted to transmit, receive ortransmit and receive data through a medium. The term communicationstransceiver or communications device is defined as any apparatus ormechanism adapted to transmit and receive data through a medium. Thecommunications device or communications transceiver may be adapted tocommunicate over any suitable medium, including wireless or wired media.Examples of wireless media include RF, infrared, optical, microwave,UWB, Bluetooth, WiMax, WiMedia, WiFi, or any other broadband medium,etc. Examples of wired media include twisted pair, coaxial, opticalfiber, any wired interface (e.g., USB, Firewire, Ethernet, etc.). Theterm Ethernet network is defined as a network compatible with any of theIEEE 802.3 Ethernet standards, including but not limited to 10 Base-T,100Base-T or 1000Base-T over shielded or unshielded twisted pair wiring.The terms communications channel, link and cable are usedinterchangeably.

The term multimedia player or device is defined as any apparatus havinga display screen and user input means that is capable of playing audio(e.g., MP3, WMA, etc.), video (AVI, MPG, WMV, etc.) and/or pictures(JPG, BMP, etc.). The user input means is typically formed of one ormore manually operated switches, buttons, wheels or other user inputmeans. Examples of multimedia devices include pocket sized personaldigital assistants (PDAs), personal media player/recorders, cellulartelephones, handheld devices, and the like.

Some portions of the detailed descriptions which follow are presented interms of procedures, logic blocks, processing, steps, and other symbolicrepresentations of operations on data bits within a computer memory.These descriptions and representations are the means used by thoseskilled in the data processing arts to most effectively convey thesubstance of their work to others skilled in the art. A procedure, logicblock, process, etc., is generally conceived to be a self-consistentsequence of steps or instructions leading to a desired result. The stepsrequire physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared and otherwise manipulated in a computer system. It has provenconvenient at times, principally for reasons of common usage, to referto these signals as bits, bytes, words, values, elements, symbols,characters, terms, numbers, or the like.

It should be born in mind that all of the above and similar terms are tobe associated with the appropriate physical quantities they representand are merely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present invention,discussions utilizing terms such as ‘processing,’ ‘computing,’‘calculating,’ ‘determining,’ ‘displaying’ or the like, refer to theaction and processes of a computer system, or similar electroniccomputing device, that manipulates and transforms data represented asphysical (electronic) quantities within the computer system's registersand memories into other data similarly represented as physicalquantities within the computer system memories or registers or othersuch information storage, transmission or display devices.

The invention can take the form of an entirely hardware embodiment, anentirely software embodiment or an embodiment containing a combinationof hardware and software elements. In one embodiment, a portion of themechanism of the invention is implemented in software, which includesbut is not limited to firmware, resident software, object code, assemblycode, microcode, etc.

Furthermore, the invention can take the form of a computer programproduct accessible from a computer-usable or computer-readable mediumproviding program code for use by or in connection with a computer orany instruction execution system. For the purposes of this description,a computer-usable or computer readable medium is any apparatus that cancontain, store, communicate, propagate, or transport the program for useby or in connection with the instruction execution system, apparatus, ordevice, e.g., floppy disks, removable hard drives, computer filescomprising source code or object code, flash semiconductor memory (USBflash drives, etc.), ROM, EPROM, or other semiconductor memory devices.

Mobile Device/Cellular Phone/PDA System

A simplified block diagram illustrating an example communication deviceincorporating the WLAN idle connection state power reduction mechanismof the present invention is shown in FIG. 3. The communication devicemay comprise any suitable wired or wireless device such as a multimediaplayer, mobile station, mobile device, cellular phone, PDA, wirelesspersonal area network (WPAN) device, Bluetooth EDR device, etc. Forillustration purposes only, the communication device is shown as acellular phone or smart phone. Note that this example is not intended tolimit the scope of the invention as the frequency reference dithermechanism of the present invention can be implemented in a wide varietyof wireless and wired communication devices.

The cellular phone, generally referenced 70, comprises a basebandprocessor or CPU 71 having analog and digital portions. The basiccellular link is provided by the RF transceiver 94 and related one ormore antennas 96, 98. A plurality of antennas is used to provide antennadiversity which yields improved radio performance. The cell phone alsocomprises internal RAM and ROM memory 110, Flash memory 112 and externalmemory 114.

The basic cellular link is provided by the RF transceiver 94 and relatedone or more antennas 96, 98. A plurality of antennas is used to provideantenna diversity which yields improved radio performance. The cellphone also comprises internal RAM and ROM memory 110, Flash memory 112and external memory 114.

Several user interface devices include microphone 84, speaker 82 andassociated audio codec 80, a keypad for entering dialing digits 86,vibrator 88 for alerting a user, camera and related circuitry 100, a TVtuner 102 and associated antenna 104, display 106 and associated displaycontroller 108 and GPS receiver 90 and associated antenna 92.

A USB interface connection 78 provides a serial link to a user'scomputer (e.g., PC, Mac, etc.) or other device. An FM receiver 72 andantenna 74 provide the user the ability to listen to FM broadcasts. WLANradio and interface 76 and antenna 77 provide wireless connectivity whenin a hot spot or within the range of an ad hoc, infrastructure or meshbased wireless LAN network. In accordance with the invention, the WLANcomprises the idle connection state power reduction mechanism asindicated in block 128. Alternatively, software adapted to implement theidle connection state power reduction mechanism may be as provided as atask (block 129) to be executed by the digital baseband processor 71.

Note that the idle connection state power reduction mechanism may beimplemented as hardware, as software executed as a task on the basebandprocessor 71 or a combination of hardware and software. Implemented as asoftware task, the program code operative to implement the idleconnection state power reduction mechanism of the present invention isstored in one or more memories 110, 112 or 114.

A Bluetooth EDR radio and interface 73 and antenna 75 provide Bluetoothwireless connectivity when within the range of a Bluetooth wirelessnetwork. Further, the communication device 70 may also comprise a WiMAXradio and interface 123 and antenna 125. SIM card 116 provides theinterface to a user's SIM card for storing user data such as addressbook entries, etc. The communication device 70 also comprises an UltraWideband (UWB) radio and interface 83 and antenna 81. The UWB radiotypically comprises an MBOA-UWB based radio.

Portable power is provided by the battery 124 coupled to powermanagement circuitry 122. External power is provided via USB power 118or an AC/DC adapter 120 connected to the power management circuitrywhich is operative to manage the charging and discharging of the battery124 and power delivery to the communication device.

Example WLAN Transceiver

A simplified block diagram illustrating an example WLAN transceiverincorporating the idle connection state power reduction mechanism of thepresent invention is shown in FIG. 6. The example circuit, generallyreferenced 130, comprises a WLAN transceiver 131 coupled to an antenna132 and a host 144. The WLAN transceiver 131 comprises a WLAN radio 134,baseband processor/PHY circuit 136, MAC 138, host interface 142,processor/controller 148, flash memory 150, SRAM 152 and EEPROM 154.

The host interface 142 functions to interface the WLAN to a host entity144. The host may comprise any suitable computing device such as a PDA,laptop computer, desktop computer, handheld telecommunications device,etc. The host interface may be adapted to communicate with the host inany manner. Typically, the host interface is adapted to communicate viaa standard interface including, but not limited to, PCI, CardBus, USB,SDIO, SDI, etc.

The medium access controller (MAC) 138 is operative to provide Layer 2functionality (i.e. the OSI model). The MAC handles communications andimplements the protocol between the host and the PHY Layer 1 hardware.The baseband processor/PHY module 50 implements the Layer 1functionality (OSI model). The PHY performs modulation and demodulationof data (i.e. OFDM in the case of WLAN 802.11a or 802.11g, or Barker andCCK in the case of 802.11b). In addition, the PHY also performs analogto digital conversion, digital to analog conversion, carrier sensing andhandles the transmission and reception of frames. The radio module 134,coupled to antenna 132, functions to perform the radio frequency (RF)processing including upconversion from intermediate frequency (IF),downconversion from IF, filtering and amplification of the RF signal.Note that alternatively, the radio may perform direct conversion and inthis case would not need IF conversion circuitry.

In accordance with the present invention, the WLAN idle connection statepower reduction mechanism of the present invention is implemented in theMAC portion of the transceiver. Note that alternatively, the mechanismof the invention may be implemented as a task executed on theprocessor/controller. In this case, the programming code forimplementing the mechanism may reside on either of memories 150, 152,154 or on the processor/controller itself. Note also that the mechanismmay be performed entirely in hardware, software or a combination ofhardware and software. Alternatively, the mechanism may be implementedentirely in the host or a portion implemented in the host and a portionin the MAC.

The WLAN transceiver also comprises a processor/controller 148, flashmemory 150, static random access memory (SRAM) 152 and electricalerasable programmable read only memory (EEPROM) 154. Note that DRAM maybe used in place of static RAM. In addition, the mesh point may notcomprise any EEPROM memory. The controller 148 is operative to providemanagement, administration and control to the MAC, PHY and radio modulesvia bus 146. The controller is also in communication with the Flash,SRAM and EEPROM memories via a separate memory bus 156 or via a singlebus 146 shared by all the modules and memory devices.

Advertisement of Beacon Transmission Delay

It was found by a series of measurements taken by the inventor that themajority of access point devices delay beacon transmission beyond thetime needed for the IEEE 802.11e Enhanced Distributed Channel Access(EDCA). This particular type of delay is referred to as the BeaconTransmission Delay (BTD). It is defined as the time from the creation ofthe Target Beacon Transmit Time (TBTT) event by the IEEE 802.11 MAChardware timer to the time the beacon packet is forwarded to the IEEE802.11 EDCA Channel Access function in the 802.11 MAC hardware. Inaccordance with the measurements taken by the inventor, the beacontransmission delay time may vary anywhere from 40 microseconds to 400microseconds in the typical access point (AP) implementation.

If the STA had knowledge of the beacon transmission delay, it coulddelay wake-up until a point in time that is much closer to the actualarrival of the beacon message, thereby reducing the power consumption ofthe STA while in the idle connection state, and increasing standby andtalk time. Therefore, in accordance with the invention, the AP isoperative to measure and advertise a beacon transmission delay time inthe beacon message. The beacon transmission delay time is advertised viaa beacon transmission delay information element (IE). The inventionprovides methods for execution in both the access point and the station.

The functionality to be added to the access point implementation willnow be described in more detail. The beacon transmission delay of theaccess point must be determined and reported to the STAs. The actualmethod used to determine the beacon transmission delay is not criticalto the invention. Illustrative examples of the methods AP manufacturersmay use to determine the beacon transmission delay are described below.

A diagram illustrating the frame format for the beacon transmissiondelay information element of the present invention is shown in FIG. 7.The beacon transmission delay IE, generally referenced 180, comprises a1-byte ID field 182 which is assigned by the IEEE Internet AssignedNumbers Authority (IANA), 1-byte length field 184 which is equal to twoand 2-byte beacon transmission delay field 186 which comprises a valueof the beacon transmission delay either measured or assessed by the APvendor.

A flow diagram illustrating the method of advertising the beacontransmission delay for use on an access point is shown in FIG. 8. First,the beacon transmission delay (BTD) is measured using any suitablemethod (step 190). Typically, the access point manufacturer implements,for example, the methods of FIGS. 9 and 10 to measure or assess thebeacon transmission delay. Once obtained, the beacon transmission delayvalue is inserted into the beacon transmission delay field of the BeaconTransmission Delay Information Element (IE) (step 192). The BeaconTransmission Delay IE is included in 802.11 MAC management frames, suchas Beacon/Probe Responses, (Re)Association Responses and NeighborReports.

As discussed above, Access Point vendors can select several methods tomeasure the Beacon Transmission Delay in their AP devices. Two examplemethods are presented below. Note that other methods of determining (viameasurement or other means) the beacon transmission delay may be usedwithout departing from the scope of the invention.

A flow diagram illustrating a first beacon transmission delaymeasurement method is shown in FIG. 9. In this first method, the minimalbeacon transmission delay is assessed based on internal knowledge of theparticular AP implementation, which is assumed known to the APmanufacturer. If the beacon transmission process is implemented insoftware, the number of CPU (i.e. processor) instructions executed fromthe moment the TBTT interrupt is received by the MAC CPU to the momentthe Beacon frame pointer is programmed into the EDCA Channel Accesshardware block registers is counted (step 200).

The number of executed CPU instructions counted is then multiplied bythe execution time of a single instruction (step 202). This yields thebeacon transmission delay value which is then advertised to the STAs andother devices in the WLAN system via beacon transmission delay IEsinserted into the beacon messages.

A flow diagram illustrating a second beacon transmission delaymeasurement method is shown in FIG. 10. The second method is a WeightedAverage Access Delay (WAAD) method. First, the WAAD value is setinitially to zero (step 210). For every TBTT, the following process isrepeated (step 212). The timestamp of the TBTT timer expiration ismarked as START_EVENT (step 214). The timestamp of the event when theBeacon frame is input to the EDCA Channel Access hardware block ismarked as END_EVENT (step 216). The Last Beacon Transmission Delay(LBTD) is set to the difference between END_EVENT and START_EVENT, i.e.LBTD=(END_EVENT_START_EVENT) (step 218).

The WAAD is updated in accordance with a decay or forgettance factor andthe previous value for the WAAD as follows in Equation 1 (step 219).

WAAD=Forgettance_Factor×WAAD+(1−Forgettance_Factor)×LBTD  (1)

where a default value for Forgettance_Factor is 0.9.

The functionality to be added to the station implementation will now bedescribed in more detail. The beacon transmission delay measured ordetermined by the access point is advertised to the STAs via specialinformation elements 180 (FIG. 7) as described supra.

A flow diagram illustrating the method of advertising the beacontransmission delay for use on a station is shown in FIG. 11. When a STAreceives a Beacon message that contains a Beacon Transmission DelayInformation Element (step 220) it first extracts the Beacon TransmissionDelay field value from the contents of the received IE. The STA thenupdates the Wake For Beacon Reception time value by adding the BeaconTransmission Delay to the TBTT time, as in Equation 2 below (step 222).

Wake_For_Beacon_Reception=TBTT_time+Beacon_Transmission_Delay  (2)

where

-   -   TBTT time is the Beacon Interval advertised by the Access Point        in Beacon frames;    -   Beacon Transmission Delay is the value of the Beacon        Transmission Delay field of the Beacon Transmission Delay        Information Element;

The STA then programs (i.e. configures) the hardware timer used toindicate that the STA needs to prepare for the Beacon reception with theupdated Wake For Beacon Reception value (step 224).

Reduction in Beacon Frame Reception Time

In order to enable maximum range coverage, Beacons are transmitted atlow rates. The typical Beacon transmission PHY rate is 1 Mbps. Thetypical Beacon length measured in different environments varies fromapproximately 130 to 300 bytes which translates to Beacon transmissiontimes of 1.2 milliseconds to 2.6 milliseconds. It has been found thatthe contents of the Beacon messages do not change significantly betweenconsecutive Beacon transmissions. An exception to this is the TSFInformation Element and TIM Information Element that do change everyBeacon period.

Changes in the contents of Beacon messages usually occur in the leading73 bytes as follows:

1. 802.11 Header: 24 bytes, the duration field typically changes.2. Timestamp: 8 bytes, changes every Beacon message.3. Beacon Interval: 2 bytes, does not change.4. Capabilities Information Element: 2 bytes, does not change.5. SSID Information Element: typically 8 bytes, does not change.6. Rates Information Element: typically 13 bytes, does not change.7. Direct Sequence (DS) Parameter Set Information Element: 3 bytes, doesnot change.8. TIM Information Element: typical for network with 64 STAs: 13 bytes,changes every Beacon.

The information elements that follow the leading 73 bytes of the Beaconmessage change infrequently, if at all. Therefore, a STA could abort thereception of the Beacon if it was able to know that it received all theinformation in the Beacon transmission that changed from the previousBeacon transmission received from the Access Point, thereby reducingpower consumption.

Thus, in accordance with the present invention, a Variable Beacon DataInformation Element is defined that is transmitted in the Beacon messageby an Access Point and received and processed by the stations. A diagramillustrating the frame format for an example beacon having a variablebeacon data information element of the present invention is shown inFIG. 12.

The Variable Beacon Data Information Element, generally referenced 242,is transmitted after the Supported Rates Information Element and beforethe DS Parameter Set Information Element in the Beacon message,generally referenced 230. The following is a description of the frameformat and fields of the Beacon message and the Variable Beacon DataInformation Element, generally referenced 230. The Beacon message 230comprises

The Beacon message 230 comprises a 24-byte 802.11 header 232, 8-bytetimestamp 234, 2-byte Beacon interval field 235, 2-byte capabilitiesInformation Element 236, 8-byte SSID Information Element, 13-byteSupported Rates Information Element 240, 8-byte Variable Beacon DataInformation Element 242, other Information Elements 244 and a 4-byteFrame Check Sequence (FCS) field. The Beacon also includes a 3-byteDirect Sequence (DS) Parameter Set Information Element and 13-byte TIMInformation Element

The Variable Beacon Data Information Element 242 comprises a 2-byte IDfield 252, 1-byte length field, 2-byte size field indicating the size ofthe variable section of the Beacon message, 1-byte checksum calculatedover the variable section of the Beacon message only and a 2-bytereserved field 259. Note that the ID field 252 value is equal to 2 thusoverlaying the Frequency Hopping Parameter Set Information Elementstructure. This is not critical as the Frequency Hopping Parameter SetInformation Element is not in use in the Direct Spread Sequence (DSSS)(i.e. 802.11 DS), High Rate DSSS (i.e. 802.11b), OFDM (i.e. 802.11a),Extended Rate PHY (ERP) (i.e. 802.11g) and High Throughput (HT) (i.e.802.11n) physical layers and therefore can be reused for systemsdeploying PHYs modified to implement the mechanisms of the presentinvention.

The length field 254 is set to 5. The Size Of Variable Section field 256indicates size of the Beacon frame contents that changed from theprevious Beacon transmission. The checksum field 258 is used to validateintegrity of received Beacon contents starting from the first byte andcontinuing to the length of Size Of Variable Section. The reserved field259 is set to zero.

The functionality to be added to the implementation of the Access Pointwill now be described. A flow diagram illustrating the method of beaconreception time reduction for use on an access point is shown in FIG. 13.This method is intended to be implemented by the Access Point during itspreparation of the Beacon for transmission.

First, the Access Point prepares a Beacon transmission buffer with allrelevant Information Elements, including the Variable Beacon DataInformation Element, that are to be transmitted in the Beacon message(step 260). The following fields are initialized to zero: the durationfield in the frame header, the TSF IE and the checksum field of VariableBeacon Data Information Element (step 262). The Access Point thencalculates the size of variable section in the following manner (step264): It first compares the contents of the current Beacon in the Beacontransmission buffer with the contents of the most recently transmittedBeacon. The comparison starts immediately after the Variable Beacon DataInformation Element and continues until the end of the Beacon message. ALast_Different_IE variable is set with the offset of the InformationElement immediately following the last Information Element that differsbetween the two Beacons. The offset is calculated starting from thefirst byte of the Beacon frame stored in the Beacon transmission buffer.

The contents of the Last_Different_IE variable is copied to the Size ofVariable Section field in the Variable Beacon Data Information Element(step 266). The checksum value on the variable portion of the Beacon,called the Temp_Checksum, is then calculated in accordance with thefollowing algorithm (step 268): The Temp_Checksum variable isinitialized to the hex value of 0xAB and the variable I is initializedto 0. For I=0 to Last_Different_IE, Temp_Checksum=Temp_Checksum XOR (thecontents of the byte read from the Beacon transmission buffer at offsetI). The contents of Temp_Checksum are copied to the checksum field 258(FIG. 12) in the Variable Beacon Data Information Element 242 (step269). The Access Point then transmits the prepared Beacon messageaccording to the rules defined in the IEEE 802.11 standard.

The following functionality is added to the implementation of theStations in the WLAN system. A flow diagram illustrating the method ofbeacon reception time reduction for use on an access point is shown inFIG. 14. This method is intended to be implemented by the STAs in theWLAN network. The STA maintains a logical variable (or flag)Previous_Beacon_Correct which is used to indicate whether the previouslyreceived Beacon was received correctly. Initially, thePrevious_Beacon_Correct variable is set to FALSE (step 270). The STAwaits for receipt of a beacon. If the Beacon received by the STA is thefirst Beacon after joining the BSS (step 272), then it receives thecomplete data set in the Beacon message (step 274). If the Frame CheckSequence (FCS) of the Beacon is correct (step 276) thenPrevious_Beacon_Correct is set to TRUE (step 278) and the method returnsto wait for the next Beacon.

If the Beacon received is not the first Beacon after joining the BSS(step 272), the STA implement the following process of Beacon receptionfor subsequently received Beacons. It is first checked whetherPrevious_Beacon_Correct is TRUE (step 280). If not then this means thatthe previous Beacon was not received correctly and the entire Beaconmessage should be received. The method continues with step 274.

If Previous_Beacon_Correct is TRUE (step 280), then only the variableportion of the Beacon needs to be received as follows. The start ofBeacon reception is detected using any suitable existing mechanism as isknown in the art (step 282). This can be achieved by detecting the frametype of the Beacon and the BSSID STA it is associated with. The STAbegins storing received Beacon data in a Beacon received data buffer.The Variable Beacon Data Information Element is then detected via itsunique ID (step 284). The contents of the Variable Beacon DataInformation Element is parsed and the Size Of Variable Section field isread (step 286).

In accordance with the present invention, Beacon reception is stoppedafter the number of bytes indicated in the Size Of Variable Sectionfield is received (step 288). Thus, the STA need not receive the entireBeacon message beyond the number of bytes called for in the Size OfVariable Section field. Once the variable portion of data is received,the STA turns off the radio thereby significantly reducing powerconsumption (step 290).

The integrity of the received Beacon is then verified in the followingmanner (step 292). The contents of the checksum field from the VariableBeacon Data Information Element is copied to a Validate_Checksumvariable. The following fields of the Beacon data buffer are initializedto zero: the duration field in the Frame Header, TSF IE and the Checksumfield of Variable Beacon Data Information Element. The contents of theVariable Section field in the Variable Beacon Data Information Elementis copied to the Last_Different_IE variable.

The Temp_Checksum value is then calculated according to the followingalgorithm (step 294): The Temp_Checksum variable is initialized to thehex value of 0xAB and the variable I is initialized to 0. For I=0 toLast_Different_IE, Temp_Checksum=Temp_Checksum XOR (the contents of thebyte read from the Beacon receive buffer at offset I).

The contents of Temp_Checksum are then compared to the contents of theValidate_Checksum variable (step 296). If the values are equal then theBeacon data is considered to have been correctly received andPrevious_Beacon_Correct is set to TRUE (step 299). Otherwise, the Beacondata is considered to have been received incorrectly andPrevious_Beacon_Correct is set to FALSE (step 298). The method returnsto step 272.

It is intended that the appended claims cover all such features andadvantages of the invention that fall within the spirit and scope of thepresent invention. As numerous modifications and changes will readilyoccur to those skilled in the art, it is intended that the invention notbe limited to the limited number of embodiments described herein.Accordingly, it will be appreciated that all suitable variations,modifications and equivalents may be resorted to, falling within thespirit and scope of the present invention.

1. A method for use in an access point in a wireless local area network(WLAN) system, said method comprising the steps of: measuring beacontransmission delay on said access point; and advertising said measuredbeacon transmission delay in messages thereby permitting recipientstations to schedule wake up times for beacon reception withsignificantly improved accuracy.
 2. The method according to claim 1,wherein said step of measuring said beacon transmission delay comprisesthe steps of: determining the number of processing instructions executedfrom receipt of a Target Beacon Transmission Time (TBTT) interrupt untilthe programming of the beacon frame pointer into the channel accessblock; and multiplying said number of processing instructions executedby an instruction execution time to yield said beacon transmissiondelay.
 3. The method according to claim 1, wherein said step ofmeasuring said beacon transmission delay comprises calculating aweighted average access delay.
 4. The method according to claim 3,wherein said weighted average access delay is calculated as a functionof a forgettance factor, previous weighted average access delay and alast beacon transmission delay, wherein said last beacon transmissiondelay comprises the time duration from Target Beacon Transmission Time(TBTT) timer expiration to when the beacon message is input to saidchannel access block.
 5. The method according to claim 1, wherein saidstep of advertising comprises inserting said measured beacontransmission delay in a beacon transmission delay information elementincluded in media access control (MAC) management frames.
 6. The methodaccording to claim 1, wherein said message comprises a message from thegroup consisting of Beacon, Probe response, Association Response,Re-association Response and Neighbor Report.
 7. A method for use in anaccess point in a wireless local area network (WLAN) system, said methodcomprising the steps of: measuring beacon transmission delay on saidaccess point from a target beacon transmission time (TBTT) event to thetime a beacon packet is forwarded to a channel access function; andadvertising said measured beacon transmission delay in beacon messagesthereby permitting recipient stations to schedule wake up times forbeacon reception with better accuracy.
 8. The method according to claim7, wherein said channel access function comprises an IEEE 802.11Enhanced Distributed Channel Access (EDCA).
 9. The method according toclaim 7, wherein said step of measuring said beacon transmission delaycomprises the steps of: determining the number of processinginstructions executed from receipt of a Target Beacon Transmission Time(TBTT) interrupt until the programming of the beacon frame pointer intothe channel access block; and multiplying said number of processinginstructions executed by an instruction execution time to yield saidbeacon transmission delay.
 10. The method according to claim 7, whereinsaid step of measuring said beacon transmission delay comprisescalculating a weighted average access delay.
 11. The method according toclaim 7, wherein said step of advertising comprises inserting saidmeasured beacon transmission delay in a beacon transmission delayinformation element included in media access control (MAC) managementframes.
 12. A method for use in an access point in a wireless local areanetwork (WLAN) system, said method comprising the steps of: determininga delay in transmission of beacons beyond a time normally required forchannel access; and advertising said measured beacon transmission delayin beacon messages thereby permitting recipient stations to schedulewake up times for beacon reception with better accuracy.
 13. The methodaccording to claim 12, wherein said step of measuring said beacontransmission delay comprises the steps of: determining the number ofprocessing instructions executed from receipt of a Target BeaconTransmission Time (TBTT) interrupt until the programming of the beaconframe pointer into the channel access block; and multiplying said numberof processing instructions executed by an instruction execution time toyield said beacon transmission delay.
 14. The method according to claim12, wherein said step of measuring said beacon transmission delaycomprises calculating a weighted average access delay.
 15. A method foruse in a station in a wireless local area network (WLAN) system, saidmethod comprising the steps of: receiving a message containing beacontransmission delay information transmitted by an access point; andupdating a wake for beacon reception time value in accordance with saidbeacon transmission delay information.
 16. The method according to claim15, wherein said beacon transmission delay information is received in aBeacon Transmission Delay Information Element (IE).
 17. The methodaccording to claim 15, wherein said step of updating comprises the stepof calculating a new wake for beacon time value as a function of saidbeacon transmission delay information and a beacon interval advertisedby said access point.
 18. The method according to claim 15, wherein saidstep of updating comprises the step of calculating a wake for beacontime value as the sum of said beacon transmission delay and a TargetBeacon Transmission Time (TBTT) advertised by said access point.
 19. Themethod according to claim 15, further comprising the step of programminga hardware wake-up timer with said updated wake for beacon receptiontime value.
 20. The method according to claim 15, wherein said messagecomprises a message from the group consisting of Beacon, Probe response,Association Response, Re-association Response and Neighbor report.
 21. Amobile communication device, comprising: a cellular radio; a WLAN radio;a processor communicatively coupled to said WLAN radio and said cellularradio, said processor operative to: receive a message containing beacontransmission delay information transmitted by an access point; andadjust a wake for beacon reception time value in accordance with saidbeacon transmission delay information.
 22. The mobile communicationsdevice according to claim 21, wherein said measured beacon transmissiondelay is transmitted in a beacon transmission delay information element.23. The method according to claim 21, wherein said processor isoperative to adjust said wake for beacon reception time value bycalculating a new wake for beacon time value as a function of saidbeacon transmission delay information and a beacon interval advertisedby said access point.
 24. The method according to claim 21, wherein saidprocessor is operative to adjust said wake for beacon reception timevalue by calculating a wake for beacon time value as the sum of saidbeacon transmission delay and a Target Beacon Transmission Time (TBTT)advertised by said access point.
 25. The method according to claim 21,wherein said processor is further operative to program a hardwarewake-up timer with said adjusted wake for beacon reception time value.